FXLS8471Q Datasheet, PDF (23/75 Pages) Freescale Semiconductor, Inc – Linear Accelerometer

8.4 Auto-Wake/Sleep mode
FXLS8471Q can be configured to transition between sample rates (with their respective current consumptions) based on the
status of the embedded interrupt event generators in the device. The advantage of using the Auto-Wake/Sleep is that the system
can automatically transition to a higher sample rate (higher current consumption) when needed but spends the majority of the
time in the Sleep mode (lower current) when the device does not require higher sampling rates. Auto-Wake refers to the device
being triggered by one of the interrupt event functions to transition to a higher sample rate. This may also interrupt the processor
to transition from a sleep mode to a higher power mode.
Sleep mode occurs when none of the enabled interrupt event functions has detected an interrupt within the user-defined, time-
out period. The device will then transition to the specified lower sample rate. It may also alert the processor to go into a lower power
mode to save power during this period of inactivity. Please refer to AN4074 for more detailed information on configuring the Auto-
Wake/Sleep function.
8.5 Freefall and Motion event detection
FXLS8471Q integrates a programmable threshold based acceleration detection function capable of detecting either motion or
freefall events depending upon the configuration. For further details and examples on using the embedded freefall and motion
detection functions, please refer to Freescale application note AN4070.
8.5.1 Freefall detection
The detection of “Freefall” involves the monitoring of the X, Y, and Z axes for the condition where the acceleration magnitude is
below a user-specified threshold for a user-definable amount of time. Typically, the usable threshold ranges are between
±100 mg and ±500 mg.
8.5.2 Motion detection
Motion detection is often used to alert the main processor that the device is currently in use. When the acceleration exceeds a
set threshold for a set amount of time, the motion interrupt is asserted. A motion can be a fast moving shake or a slow moving
tilt. This will depend on the threshold and timing values configured for the event. The motion detection function can analyze static
acceleration changes or faster jolts. The timing value is set by a configurable debounce counter. The debounce counter acts like
a filter to indicate whether the condition exists for longer than a set amount of time (that is, 100 ms or longer). There is also
directional data available in the source register to detect the direction of the motion that generated the interrupt. This is useful for
applications such as directional shake or flick detection, and can also assist gesture detection algorithms by indicating that a
motion gesture has started.
8.6 Transient detection
FXLS8471Q integrates an acceleration transient detection function that incorporates a high-pass filter. Acceleration data goes
through the high-pass filter, eliminating the DC tilt offset and low frequency acceleration changes. The high-pass filter cutoff can
be set by the user to four different frequencies which are dependent on the selected Output Data Rate (ODR). A higher cutoff
frequency ensures that DC and slowly changing acceleration data will be filtered out, allowing only the higher frequencies to pass.
The transient detection feature can be used in the same manner as the motion detection by bypassing the high-pass filter. There
is an option in the configuration register to do this. This adds more flexibility to cover the various customer use cases.
Many applications use the accelerometer’s static acceleration readings (that is, tilt) which measure the change in acceleration
due to gravity only. These functions benefit from acceleration data being filtered with a low-pass filter where high-frequency data
is considered noise. However, there are many functions where the accelerometer must analyze dynamic acceleration. Functions
such as tap, flick, shake and step counting are based on the analysis of the change in the dynamic acceleration. The transient
detection function can be routed to either interrupt pin through bit 5 in CTRL_REG5 register (0x2E). Registers 0x1D – 0x20 are
used for configuring the transient detection function. The source register contains directional data to determine the direction of
the transient acceleration, either positive or negative. For further information of the embedded transient detection function along
with specific application examples and recommended configuration settings, refer to Freescale application note AN4461.
8.7 Pulse detection
FXLS8471Q has embedded single/double and directional pulse detection. This function employs several timers for programming
the pulse width time and the latency between pulses. The detection thresholds are independently programmable for each axis.
The acceleration data input to the pulse detection circuit can be put through both high and low-pass filters, allowing for greater
flexibility in discriminating between pulse and tap events. The PULSE_SRC register provides information on the axis, direction
(polarity), and single/double event status for the detected pulse or tap. For more information on how to configure the device for
pulse detection, please refer to Freescale application note AN4072.
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FXLS8471Q
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